Philosophical Magazine Letters
ISSN: 0950-0839 (Print) 1362-3036 (Online) Journal homepage: http://www.tandfonline.com/loi/tphl20
Hydrogen-assisted damage in austenite/ martensite dual-phase steel Motomichi Koyama, Cemal Cem Tasan, Tatsuya Nagashima, Eiji Akiyama, Dierk Raabe & Kaneaki Tsuzaki To cite this article: Motomichi Koyama, Cemal Cem Tasan, Tatsuya Nagashima, Eiji Akiyama, Dierk Raabe & Kaneaki Tsuzaki (2016): Hydrogen-assisted damage in austenite/martensite dual-phase steel, Philosophical Magazine Letters, DOI: 10.1080/09500839.2015.1130275 To link to this article: http://dx.doi.org/10.1080/09500839.2015.1130275
Published online: 04 Jan 2016.
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Date: 05 January 2016, At: 04:11
Philosophical Magazine Letters, 2015 http://dx.doi.org/10.1080/09500839.2015.1130275
Hydrogen-assisted damage in austenite/martensite dual-phase steel Motomichi Koyamaa, Cemal Cem Tasanb, Tatsuya Nagashimaa, Eiji Akiyamac, Dierk Raabeb and Kaneaki Tsuzakia,c a
Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung GmbH, MaxPlanck-Straβe 1, 40237 Düsseldorf, Germany; cResearch Center for Strategic Materials, National Institute for Materials Science, 1-2-1, Sengen, Ibaraki 305-0047, Japan
Downloaded by [Motomichi Koyama] at 04:11 05 January 2016
b
ABSTRACT
For understanding the underlying hydrogen embrittlement mechanism in transformation-induced plasticity steels, the process of damage evolution in a model austenite/martensite dual-phase microstructure following hydrogenation was investigated through multi-scale electron channelling contrast imaging and in situ optical microscopy. Localized diffusible hydrogen in martensite causes cracking through two mechanisms: (1) interaction between {1 1 0}M localized slip and {1 1 2}M twin and (2) cracking of martensite– martensite grain interfaces. The former resulted in nanovoids along the {1 1 2}M twin. The coalescence of the nanovoids generated plate-like microvoids. The latter caused shear localization on the specific plane where the crack along the martensite/martensite boundary exists, which led to additional martensite/martensite boundary cracking.
ARTICLE HISTORY
Received 17 August 2015 Accepted 6 December 2015 KEYWORDS
Embrittlement; hydrogen in metals; damage; electron channelling; Fe-based alloys; silver decoration
1. Introduction Understanding hydrogen embrittlement (HE) mechanisms is the most important bottleneck in the practical application of high-strength steels in components exposed to severe operating conditions. Knowledge of the HE mechanisms is especially crucial for martensitic steel [1,2], dual-phase steel [3,4], twinning-induced plasticity steel [5,6] and transformation-induced plasticity (TRIP) steel [7,8]. In specific cases, the actual HE mechanism has been elucidated by combining theories associated with various hydrogen effects [2,4,9], although several uncertainties remain. The difficulty of clarifying the HE mechanism arises from the complexity of microstructure-driven heterogeneities: hydrogen is often segregated in various trap sites and the associated damage evolution is strongly localized. The inhomogeneity is particularly complicated in high-strength multiple-phase steels, such as ferrite/martensite [4] and austenite/martensite microstructures [7,8,10]. The most complex example is TRIP steel [7,8,10], which contains thermally or mechanically induced martensite and retained austenite. Moreover, the martensite was reported to contain transformation twins [8], which CONTACT Motomichi Koyama © 2015 Taylor & Francis
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are observed in thin plate [11] and lenticular martensite [12]. The full understanding of HE in this particular microstructure requires careful consideration of the roles of the two different phases, viz. martensite and retained austenite. Although martensite in TRIP steel is known to be susceptible to HE [7,13], the underlying embrittlement mechanism is not yet fully understood. A crucial factor preventing understanding of HE is the occurrence of deformation-induced martensitic transformation [10]. Since martensitic transformation occurs successively with plastic strain, hydrogen-assisted cracking is difficult to identify owing to the sequential evolution of the underlying microevents. A specific question in that context is if the cracking occurred within the martensite or whether instead the cracking induced deformation-induced martensite formation at the crack tips via the TRIP effect. In a dual-step effort to resolve this question, we present here the first investigations of HE in a stable austenite/martensite dual-phase microstructure. More specifically, we discuss effects of slip localization and transformation twins on damage evolution behaviour, which helps to understand underlying mechanism of HE in the TRIP steels. Observations of the phenomenon in steel with reduced austenite stability will follow in a separate report.
2. Experimental A Fe–32.0Ni–0.19C–0.005Mn–0.004Si–
